second the shotgun over pistol sentiments presented here, even smaller rounds are dangerous. 9 mm are smaller, but from what I have heard they can get even more house penetration than the large caliber guns. It's because the round is long and thin, so as long as it hits something soft (like human tissue or drywall) it just tunnels through and keeps going.Glaser safty rounds are your answer. The minute they impact anything, they fragment. The added benefit, when they hit flesh, the damage is much worse then if hit by a solid slug. With these, you won't kill your neighbor. I keep 5 in my Beretta 9mm.
Peace Out~